Flow, level & control
Vortex meters make use of a principle called the von Kármán effect
Installation Requirements Planning a flow meter installation starts with knowing the line size, pipe flow direction, material of construction, and flange-pressure rating. Complications due to equipment accessibility, valves, regulators, and available straight pipe run lengths should also be identified. Many types of flow meters must be installed with a run of straight pipe before and after their mounting location. Where this is not possible, a flow conditioner can be used to isolate liquid flow disturbances from the flow meter while minimising the pressure drop across the conditioner.
Below: Ultrasonic meters have no moving or wetted parts
Desired Accuracy In many production processes, accurate flow measurements can be the difference between on- spec quality and wasted product. Flow meter accuracy is specified in percentage of actual reading (OR), percentage of calibrated span (CS), or percentage of full-scale (FS) units. It is normally stated at minimum, normal, and maximum flow rates. A clear understanding of these requirements is needed for a meter’s performance to be acceptable over its full range.
Application Environment Flow meters can be employed under a host of varying conditions in a chemical process plant. For example, users must decide whether the low or high flow range is most important for their metering application. This information will help in sizing the correct instrument for the job. Pressure and temperature conditions are equally important process parameters. Users should also consider pressure drop (the decrease in pressure from one point in a pipe to another point downstream) in flow measurement devices, especially with high- viscosity fluids. In addition, viscosity and density may fluctuate due to a physical or temperature change in the process fluid.
Fluid Characteristics Users should be cautious that the selected flow meter is compatible with the fluid and conditions they are working with. Many chemical and petrochemical operations involve abrasive or corrosive fluids, which move under aerated, pulsating, swirling or reverse-flow conditions. Thick and compacted materials can clog or damage internal meter components, hindering accuracy and resulting in frequent downtime and repair.
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Power Availability and Hazardous Areas Pneumatic instrumentation was once used in most hazardous area applications, since there was no power source to cause an explosion. Today’s ATEX installations normally call for intrinsically safe instruments, which are “current limited” by safety barriers to eliminate a potential spark or Explosion proof solutions which feature approved E xd enclosures. Self- powered flow meters offer an advantage in such environments or where a power source is not available. They do not require external power to provide a local rate/total indicator display for a field application, and instead rely on battery- power for displaying, logging and transmitting data. Solar-powered systems can also be used in remote areas without power.
Necessary Approvals UK firms are obliged to comply with strict standards set by the EU. Approvals for the use of flow measurement equipment in hazardous locations are covered by the ATEX directive. Standards such as the Measuring Instruments Directive (MID) apply to fiscal and custody transfer metering for liquids and gases. In terms of environmental emissions, industrial flow meters must meet the Electromagnetic Compatibility (EMC) Standards EN55011:1992 and EN61326-1:1997.
Output/Indication Flow meter users must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analogue, digital, or shared. The choice of a digital communications protocol such as HART, FOUNDATION Fieldbus or Modbus also figures into this decision. In a large industrial facility, flow readings are typically supplied to an industrial automation and control system or SCADA system for use in process control and optimisation strategies.
OTHER IMPORTANT CONSIDERATIONS Companies purchasing flow meters should remember that accurate instruments cost more based on their capabilities. It is better to locate the type of meter suited to a specific application before sacrificing features for cost savings. Users should closely evaluate their process conditions, including flow rates, pressure and temperature,
and operating ranges. Do not be swayed by lower priced alternatives that would be applied outside of their capabilities. All flow meters are affected to some extent by the flowing medium they are metering and by the way they are installed. Consequently, their performance in real world conditions will often be different from the reference conditions under which they calibrated. For the lowest uncertainty of measurement, positive displacement meters are generally the best option. Electromagnetic meters provide for the some of the widest flow “turn-down” ranges and turbine meters are usually the best choice for the highest short- term repeatability vs price. Despite their high initial cost, Coriolis meters are ideal for measuring particularly viscous substances and anywhere that the measurement of mass rather than volume is required. Flow meter users should also take care to
examine long-term ownership costs. A flow meter with a low purchase price may be very expensive to maintain. Alternatively, a meter with a high purchase price may require very little service. Lower purchase price does not always represent the best value. Generally speaking, flow meters with few or
no moving parts require less attention than more complex instruments. Meters incorporating multiple moving parts can malfunction due to dirt, grit or grime present in the process fluid. Meters with impulse lines can also plug or corrode, and units with flow dividers and pipe bends sometimes suffer from abrasive media wear and blockages. Changes in temperature also affect the internal dimensions of the meter and require compensation. The need to recalibrate a flow meter depends
on how well the instrument fits a particular application. If the application is critical, meter accuracy should be checked at frequent intervals. Otherwise, recalibration may not be necessary for years because the application is non-critical, or there are no factors which will change the meter’s performance. No matter the chosen flow meter technology,
overall system accuracy will not exceed that of equipment used to perform the meter calibration. The most precise flow calibration systems on the market employ a positive displacement design. This type of calibrator, usually directly traceable to National standards, provides total accuracy of at least 0.05 per cent.
SUMMARY Choosing the right flow measurement solution can have a major impact on operational and business performance. For this reason, companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. The effort spent learning about basic flow measurement techniques, and available meter options, will ensure a successful application once the equipment is installed.
Bell Flow Systems
www.bellflowsystems.co.uk October 2019 Instrumentation Monthly
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